The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Shock absorbers are typically used in conjunction with automotive suspension systems or other suspension systems to absorb unwanted vibrations that occur during movement of the suspension system. In order to absorb these unwanted vibrations, automotive shock absorbers are generally connected between the sprung (body) and the unsprung (suspension/drivetrain) masses of the vehicle.
Typical passive shock absorbers provide the same magnitude of damping force regardless of the frequency of the input. For a given input velocity, the damping force generated by a conventional passive shock absorber remains the same regardless of the frequency of the input. Typically, the primary ride frequency of a passenger vehicle is in the range of 1 to 2 Hertz. When a vehicle goes over a road surface with a lower frequency input, a higher amount of damping is preferred to manage the road inputs. During handling events (where directional stability is critical), a higher amount of damping is also preferred. For example, the vehicle may be subjected to body roll during handling events. The frequency of body roll in a typical passenger vehicle commonly ranges from 2 to 4 Hertz depending on the roll-stiffness and the height of the center of gravity of the vehicle. When the damper system experiences larger excitation forces, higher damping forces are required. When conventional passive shock absorbers are used, the higher damping forces result in more harshness and a decrease in ride quality.
Active shock absorbers change the damping of the shock absorber in real-time to address different vehicle suspension inputs. There are many types of active shock absorbers. One type of active shock absorber utilizes an electro-magnetic actuator that applies a magnetic force to a piston rod of the shock absorber independent of the damping forces generated by the compression and rebound valving. Such electro-magnetic actuators typically comprise a combination of permanent magnets and a plurality of coils that are co-axially arranged with one another. The permanent magnets may be mounted to the outer tube of the shock absorber and the plurality of coils may be coupled to the piston rod or vice versa. When electricity is supplied to the plurality of coils, the plurality of coils create an electro-magnetic field that interacts with the magnetic field of the permanent magnets and applies a magnetic force to the piston rod. The magnetic force effectively increases or decreases the damping force of the shock absorber, either firming up or softening the suspension.
Unlike passive shock absorbers, electro-magnetic shock absorbers can generate damping forces independently of the velocity of the piston rod inputs. As a result, large excitation forces do not require more hydraulic damping from the shock absorber and therefore do not introduce increased harshness. This is a major advantage of electro-magnetic shock absorbers because it resolves the trade-off in hydraulic damper systems between primary body control (which requires large damping forces) and secondary comfort (which requires low damping forces). Although active shock absorbers can provide ride and handling improvements, they are considerably more expensive than traditional passive shock absorbers due to the high cost of the electro-magnetic materials used in the electro-magnetic actuator. Electro-magnetic shock absorbers are also expensive because they typically require a shock absorber to be re-designed to accommodate the space required for the permanent magnets and plurality of coils of the electro-magnetic actuator.